This invention relates to a method for operating a spark-ignition internal combustion engine with direct injection, in which the fuel is injected into a combustion chamber in a plurality of part amounts, predominantly in the compression stroke.
When spark-ignition internal combustion engines with direct injection operate, a series of measures are performed to improve the formation of mixture, so that reliable ignition is brought about in stratified charge operating modes of the internal combustion engines. Attempts are often made to influence the mixture formation in a targeted fashion by clocking the fuel injection.
German Patent DE 198 577 85 C2 discloses a method for mixture formation in a combustion chamber of an internal combustion engine in which a three-stage fuel injection is performed without interrupting the injection and in which a main amount of fuel is connected to an ignition amount of fuel via a supplementary amount of fuel.
In the known methods, an optimum combustion method is often not achieved in modern spark-ignition internal combustion engines with direct injection because it is not possible to ensure an operating behavior of the internal combustion engine without misfires.
The object of the invention is, in contrast with the above, to configure the injection process in such a way that an ignitable mixture cloud is formed in the vicinity of an ignition source in order to bring about an improved operating behavior without misfires.
This object is achieved according to the invention.
In a first method according to the invention, in a stratified charge operating mode of the internal combustion engine, fuel injection is configured in such a way that a first, a second, and, optionally, a third part amount are introduced into the combustion chamber in a compression stroke of the internal combustion engine, and the ignition of the fuel/air mixture which is formed is performed after the part amount which is introduced last has ended. The injection of the final part amount is started at a crank angle position of −2° CA to 20° CA before the ignition. That is to say, the ignition time can be performed 2° CA before the start of the injection or 2° CA before the injection end of the final part amount until 20° CA after the start of the injection of the final part amount. The injection of the final part amount is preferably started at a crank angle position of from 5° CA to 20° CA before the ignition. In the context of the present invention, the final part amount is either the second part amount, if the overall fuel amount is introduced in two part amounts, or the third part amount, if the overall fuel amount is introduced in three part amounts. It is conceivable for the first part amount to be performed in the intake stroke preferably between 200° CA and 300° CA before a top dead center. As a result of the second and optionally third part amounts, an ignitable fuel/air cloud is subsequently formed near the spark plug before the ignition, by way of which fuel/air cloud the ignition of the complete mixture can be brought about reliably. The position and properties of the ignitable mixture cloud are influenced positively by the injection time of the final part amount, as a result of which a stratified charge operating mode can be configured in an optimum way in predominantly all load ranges. As a result, it is possible to minimize the undesired misfires further.
In one refinement of the invention, a period between the start of injection of the second part amount and the end of injection of the first part amount is approximately 0.1 to 1 ms. Here, a period is defined between the injection end of the second part amount and the ignition time or between the injection end of the third part amount and the ignition time, with the result that the positions of the first and the second or the third injections are defined by a time interval from the start of injection or from the end of injection of the second or the third injection.
According to a further refinement of the invention, a period between the start of injection of the third part amount and the end of injection of the second part amount is approximately 0.15 to 1 ms. In particular in the case of high loads, for example with an effective average pressure Pme greater than 3 bar, it is possible for the mass of fuel of the first part amount to be between 10% and 70% of the entire mass of fuel.
In a further refinement of the invention, an injection nozzle which opens to the outside is used, so that the fuel from the fuel injection nozzle is injected into the combustion chamber in the form of a hollow cone. Here, an ignitable mixture cloud is provided in approximately all of the rotational speed ranges and load ranges, with the result that misfires are prevented from occurring.
Further features and feature combinations will emerge from the description. Specific exemplary embodiments of the invention are illustrated in a simplified form in the drawings and are explained in more detail in the following description.